A mobile radio receiver, such as a FM car radio, has to change reception frequency when moving from one broadcasting area/region covered by one transmitter to another broadcasting area/region covered by another transmitter. To avoid cross interference, adjacent FM-radio broadcasting transmitters have to transmit with different frequencies. To be able to switch to the correct new reception frequency, the receiver will usually perform test receptions of possible alternative reception frequencies to thereby determine which one to switch to. To avoid irritating interruptions of the audio reception, test receptions of alternative reception frequencies will preferably take place when the received audio level is low. This will in most cases ensure that a listener does not notice these short test receptions during short silences in speech or music. When the receiver has determined which reception frequency to change to, a switch can be performed when the audio level is low. A listener will then in most cases not even notice that a change in reception frequency has taken place.
To increase the audio quality a new digital audio broadcasting (DAB) system has emerged. DAB uses coded orthogonal frequency division multiplexing (COFDM) modulation and was primarily intended as a single frequency network (SFN). In a single frequency network a DAB-radio receiver does not have to switch reception frequency when travelling between regions covered by different transmitters as they all transmit the same programmes/information with the same frequency. DAB has also made it possible to easily transfer information other than audio-information in a digital format to one or more end users.
Even though the DAB system was primarily intended as a single frequency network, there has evolved DAB regions with different transmission frequencies, each of which is possibly a SFN with a plurality of transmitters. So even in a DAB system there is a need to be able to switch/changeover reception frequency without the involvement of the listener or information end user. However, due to the continuous nature of the digital data stream the receiver can not be used for test reception of an alternative reception frequency without interrupting the user data transmission. A DAB system can loose some sub-carriers, due to example fading, some of the time but the data stream cannot be cut off for a time period necessary for a test reception of alternative frequencies. One solution to this is to use the available time during a NULL symbol to test receive alternative reception frequencies. This is possible due to the mostly available transmitter identification information (TII) transmitted with a low power level during at least some NULL symbols.
A new broadcasting system, digital video broadcasting (DVB), has emerged which is primarily intended as a video broadcasting system. DVB just as DAB can also be used to transfer digital information with an arbitrary content to an end user. The terrestial version of DVB, i.e. DVB-T, just as DAB, is intended as a single frequency network using a orthogonal frequency division and multiplex (OFDM) method. However, unlike DAB, DVB-T does not have the possibility to test receive alternative reception frequencies during a NULL symbol. A possible solution would be to use two reception chains, i.e. using two tuners, letting one be used to receive a data stream from a currently used reception frequency while the other tuner is used for test receptions of alternative reception frequencies. Unfortunately this will double the necessary hardware and thus the needed space, power and costs.